There are several reports of studies that suggest an association between exposure to ionizing radiation and the subsequent development of breast cancer. Despite the dramatic increase in thyroid cancer after radiation exposure from the Chernobyl accident in 1986, especially in young children, there are no reports of studies of women in the same exposed population of the incidence of breast cancer, or of additional risks for the development of breast cancer. Like the thyroid, the human breast concentrates iodine and women near Chernobyl were exposed to much higher than normal levels of radioiodine and other radioactive isotopes, especially cesium and strontium. Moreover, as the breast develops and changes throughout various points in a woman's life (in utero, during puberty, pregnancy and lactation), there are distinct periods of time when the breast is more susceptible to environmental exposures. These observations, together with previous evidence linking ionizing radiation to breast cancer suggests that exposure to the nuclear radiation at Chernobyl will likely result in an increase In breast cancer, especially among particularly vulnerable subpopulations, such as women who were in utero, entering puberty, pregnant or lactating at the time of the accident or who harbor a particular genetic predisposition to the disease. Hence an investigation of the role of genetic and lifestyle factors associated with nuclear radiation in women exposed to the Chernobyl fallout is warranted. Such an investigation will help us further understand radiation-induced breast cancer and factors that may modify that relationship. This R03 application proposes the following objectives in preparation for a subsequent study to evaluate breast cancer in an Eastern European population exposed to sustained high doses of ionizing radiation in 1986: 1. Initiate and establish relationships with investigators in Belarus in order to set up the infrastructure for the pilot study and the resulting R01 study; 2. Link the Belarusian Radiation Exposure database to the cancer registry database in order to identify cases associated with various levels of radiation exposure. A subset of these cases will serve as the pilot study. Age and residence matched healthy women identified through the Radiation Exposure database will serve as controls; 3. Establish and test the study infrastructure, including instrument design, specimen acquisition, and laboratory testing; and 4. Generate pilot data in support of an R01 application. By the end of this two year period, we will have put into place all the necessary mechanisms to conduct a large-scale population based study, and we will have gathered pilot data to support an R01 application investigating nuclear radiation exposure and breast cancer.